The present invention is related to a method for preparing and screening catalysts. In particular, the present invention is related to a combinatorial method for preparing and screening catalysts.
Combinatorial methods have been used extensively in the pharmaceutical industry. These methods can be an efficient and rapid way to synthesize and screen numerous different substances on a microscale. Combinatorial methods represent a systematic way to screen for potential drugs, catalysts and materials. Due to the miniaturization of the reaction with combinatorial chemistry, there are typically problems in translating reaction conditions and parameters from a microscale reaction to a corresponding macroscale reaction.
Heterogeneous catalysts have been found to be particularly useful in solid state reactions. In particular, metal alloys and elemental metals are used as heterogeneous catalysts to synthesize a variety of materials. However, most metal alloys and elemental metals are prone to oxidation. Thus, methods used to synthesize catalysts from metal alloys and elemental metals must be practiced in a controlled environment.
A multitude of combinations of metal alloys and elemental metals may be used to form catalysts. Therefore, the study of potential catalysts in the vast array of metal alloys and elemental metals may be a slow and tedious process. Due to the inefficiency of typical methods, new methods to discover catalysts are constantly being sought.
The present invention provides a combinatorial method for the preparation and screening of catalysts. In an exemplary embodiment, the method includes the steps of:
(I) providing a library of elemental catalysts;
(II) reacting the catalysts with a carbon source to form product directly on the catalyst; and
(III) screening the products to evaluate the catalysts.
A combinatorial method has been discovered which enables rapid synthesis and screening of catalysts. The combinatorial method for the discovery of catalysts is a microscale reaction. The miniaturization of the reaction enables virtually any number of different catalysts to be screened at once making it an efficient method for discovery of new catalysts. The small scale of the reaction can have environmental benefits due to the amount of chemicals used which is usually in units of micrograms. It is both a faster and a cleaner way to do experiments in search of catalysts. xe2x80x9cCatalystsxe2x80x9d as used herein refer to elemental metals, metal alloys, or combinations thereof which are effective at catalytic levels for converting a reactive substrate to a product. In particular, the combinatorial method is used for discovering catalysts used for making carbon fibrils by a heterogeneously catalyzed process.
Carbon fibrils are microscopic fibers of carbon typically having a diameter in a range between about 1 nanometer and about 500 nanometers. In particular, it is preferable to synthesize carbon fibrils with a diameter in a range between about 10 nanometers and about 50 nanometers. The aspect ratio of length of the carbon fibril to the diameter of the carbon fibril is typically greater than about 100.
Combinatorial methods used for synthesizing catalysts for carbon fibril formation include a thin film catalysts library and a powder catalysts library. xe2x80x9cLibraryxe2x80x9d as used herein refers to two or more different catalysts placed on a substrate. The catalysts may be deposited on the substrate sequentially or preferably, simultaneously. xe2x80x9cSubstratexe2x80x9d as used herein refers to any material which supports a large collection of catalysts. There is typically a minimum interaction between the supported catalysts and substrate material during chemical reaction or synthesis. However, certain substrates which have been found to be catalytic substances may have a synergistic effect on the production of carbon fibrils. Typical substrates include ceramics, for example, alumina; glass; metals, for example, aluminum, stainless steel, copper, silver, gold, platinum, and brass; and single crystals, for example, quartz, magnesium oxide, silicon, sapphire, and lanthanum aluminate.
In a preferred embodiment, a thin film catalysts library is produced using a multiple gun sputtering deposition system. The multiple gun sputtering deposition system contains elemental metal or metal alloy source placed in each gun cavity. An electrical discharge can be created at each source by applying radio frequency (RF) or direct current (DC) power in a range between about 10 Watts and about 1,000 Watts through the sputter gun, which heats the elemental metal or metal alloy to form a metal plasma vapor. The metal vapor from the sputter gun is deposited onto the counter-facing substrate. The rate of the material deposition is dependent on the level of power input. The amount of material deposited can be altered by changing the amount of time the sputter gun is powered. By coupling thin film deposition from different sputter guns with different masking patterns from an array of deposition masks, a matrix library of thin film catalysts is created. Due to the multiplicity of the number of guns and hence, elemental metals and metal alloys which can be used, the possible compositions and stoichiometry of metals which are deposited on the substrate are countless thus allowing for exploration of a vast experimental space. With multiple sputtering guns, any combination of metals can be deposited on a substrate to form a thin film catalysts library.
In various embodiments, the thin film catalysts library is built with an in-vacuum feed-in system. This enables the metal alloy library to be made without breaking the vacuum to change sources and masks for the next deposition, which keeps the metals in an atmospherically controlled environment. In particular, the in-vacuum feed-in system is filled with a gas, for example, argon, helium, nitrogen, hydrogen, and mixtures thereof. The gas in the thin film catalysts library is hereinafter referred to as xe2x80x9csputtering gasxe2x80x9d. The in-vacuum feed-in system increases the speed in the generation of libraries, and also prevents the formation of metal oxides from elemental metals and alloys which are sensitive to oxygen. Typically, the prevention of oxidation of the elemental metals and alloys is a concern but the in-vacuum feed-in system substantially inhibits the oxidation of metals and metal alloys.
Once the metal vapors are deposited on the substrate, the thin film catalysts library is typically thermally annealed. The library is heated to a temperature in a range between about 200xc2x0 C. and about 1100xc2x0 C., and preferably, to a temperature in a range between about 600xc2x0 C. and about 800xc2x0 C. The library is also typically in a non-organic gas environment to substantially prevent the oxidation of the elemental metals or metal alloys. Examples of typical gases include argon, helium, nitrogen, hydrogen and mixtures thereof. Although the invention is not dependent on theory, it is believed that the temperature and atmospheric conditions promote the interdiffusion of the combined metals to form catalysts.
An alternative manner for creating a catalysts library is through the use of a multiple channel liquid dispensing system. Each of an array of liquid dispensers can be individually controlled and programmed to dispense a liquid material. In preferred embodiments, the liquid dispensers are each filled with a soluble metal precursor such as a nitrate, acetate, or other aqueous soluble metal salt compound. An elemental metal, metal alloy or mixture thereof is carried in a soluble precursor. Once the soluble precursor or combination of soluble precursors comprising the elemental metal, metal alloy or combination thereof is deposited on the substrate as a liquid, the library is typically dried, calcined in air, and annealed in nitrogen, argon, helium, hydrogen, or combinations thereof to form an oxide-containing powder catalyst library. To synthesize metal or alloy materials from such oxide-containing powders, reducing (for example, using hydrogen, charcoal, or carbon monoxide) in a time range between about 0.5 hours and about 12 hours at a temperature in a range between about 300xc2x0 C. and about 800xc2x0 C. is found to be sufficient for most miniaturized samples in the library. By use of the soluble precursor, the oxidation of any elemental metal or alloy is not a problem.
Once the catalysts are formed, the library is placed in a suitable reactor, such as a chemical vapor deposition reactor. Typically, the catalysts on the substrate are placed in a reaction chamber, such as a fixed bed quartz tube reactor, at a temperature in a range between about 300xc2x0 C. and about 1000xc2x0 C., and preferably in a range between about 400xc2x0 C. and about 700xc2x0 C. Commonly, the reactor is initially filled with a non-organic gas in order to create a non-reactive atmosphere in the reaction chamber. In particular, the reactor is filled with a non-organic gas in order to create a non-oxidative environment such that the metal catalysts will not be oxidized. Typically, argon gas and hydrogen gas are used and are present in a volume ratio in a range between about 5.5:1 and about 1:1. A mixture of argon and hydrogen is most commonly used at a volume ratio of about 5:1 argon to hydrogen. When the organic vapor or reactant product of the organic vapor and non-organic gas under the process conditions come in contact with the catalysts, carbon fibrils may be synthesized. Due to the varying volume capacity of different reaction chambers, the flow rate can vary. Typically, the flow rate of the gas is such that it takes approximately 8 minutes to refresh the gas in the tube.
The reactor can be operated under a pressure in a range between about 1 torr and about 100 atmospheres. The synthesis is typically run at a pressure in a range between about 100 torr and about 10 atmospheres.
A carbon source (e.g., organic vapor) is released in the reactor and product, such as carbon fibrils, is formed directly on the library. Examples of organic vapors typically used in a chemical vapor deposition reactor include acetylene, ethylene, methane, benzene, carbon monoxide or mixtures thereof. Commonly, the organic vapor is mixed with one of the gases used to create a non-oxidative environment as mentioned above. Typically, a combination of ethylene gas with hydrogen gas is used at a volume ratio of 5:1. The flow rate of the vapor is such that it takes approximately 8 minutes to refresh the vapor in the tube. Carbon fibrils may also be synthesized by the reactant product of the gas and the organic vapor thereof under the process conditions. The formation of the product directly on the catalyst surface automatically accelerates the process of discovering catalysts. In addition, all the samples are integrated on the same library and may be simultaneously analyzed.
Screening for identity of product and efficiency of product formation can be done in several different ways. Examples include optical microscopy, electron microscopy, such as transmission electron microscopy and scanning electron microscopy, laser profilometry, X-ray diffraction, Raman scattering and high throughput x-ray diffraction. In particular, scanning electron microscopy is useful since yield and morphology may be assessed to identify promising catalysts.
By using combinatorial methods to discover heterogeneous catalysts, the rate of discovery compared to conventional methods may be orders of magnitude faster. Instead of examining each catalyst independently, a multitude of catalysts can be examined simultaneously. In addition, the controlled environment in which the catalysts are made ensures that the metals, metal alloys, and combinations thereof are not oxidized and the desired product can be obtained.